Cathode ray tube



rienda Api-...18, 19441;l

CATHODE RAY TUBE Clinton J. T. Young, Cambridge, Mass., assiznor to Polaroid Corporation, Dover, Del., a corporation of Delaware Application November 13, 1941, Serial No. 418,878

ii Claims.

This invention relates to cathode ray tubes, and more particularly to means for improving the definition of images produced on the screen of a cathode ray tube, as for example in a television receiver.

In one form of television reeiving apparatus, a picture is reproduced by means of a cathode ray tube having a uorescent screen formed on the inner surface of part thereof. In such an arrangement, some of the light emitted from the screen is reected back again on to the screen from the outer surface of the wall of the tube,

which results in substantially diminishing the denition of the desired image. It is a primary object of this invention to provide means for overcoming loss of definition caused in this manner.

Another object of the invention is to provide an improved cathode ray tube wherein` light emitted from the screen and reflected within the wall of the tube is prevented from falling again upon the screen.

A.` further object is to provide simple means which may be combined with conventional cathode ray tubes and will result in preventing the above loss of denition in the image produced on the screen of said tubes.

Other objects and advantages will 'in part ana pear and in part be pointed out in the course of the following detailed description oi' one or more embodiments thereof, which are given as nonlimiting examples, in connection with the accompanying drawing, in which:

Figure l is a partially diagrammatic view in section showing part of a cathode ray'tube constructed in accordance with an embodiment of the invention;

Figure 2 is an enlarged, fragmentary view in section illustrating the operation ci a conven-1 tional cathode ray tube;

Figure 3 is a view similarI to Fig. 2 showing the operation of a modication of the embodiment of the invention shown in Fig. 1; and

Figure 4 is a view similar to Fig. 1 showing a 45 further modication oi the invention.

Referring to Fig. l, element it represents a conventional glass cathode ray tube whose end wall i2 is provided 'on its inner face with a fiuorscent screen' M. In operation, any conventional form of ray generating apparatus (not shown) directs rays againstscreen i4, and thereby produces the desired image thereon. For example, if a ray strikes screen i4 at point l5, said point is thereby calmed to emit light in all di- (Cl. Z50-164) rections and accordingly becomes a bright spot in the image being reproduced on screen I4.

Now referring to Fig. 2. it will be seen that in the conventional type of cathode ray tube., the courses of light rays emitted from point l5 vary depending upon their initial directions. Thus with rays such as rays I6, which are propagated in such directions that they are incident upon the outer surface of wall l2 at angles approaching the normal, only a small amount of light is reflected back to the screen. The rest continues in substantially the same directions, with the exception of such refraction as takes place as it passes from the glass into the air. Rays such as rays i8, on the other hand, which are propagated in such directions that they are incident upon the external surface of wall I2 at angles approaching or exceeding the critical angle, suier considerable internal reflection and are directed back on to screenA il, as is indicated at points ita. Furthermore, they will continue to be renected further,v asis indicated by the dotted lines leadingV from points Na, until they are ultimately dissipat'ed: The eect ofthis internal reection in the present example is to produce a ring or series of rings of light around point l5. Since this effeci; is repeated for every other point on screen i4 from which light rays are emitted, it will be seen that the' result is a substantial loss in dennition and contrast in the" image on said screen. The present invention is directed to means for preventing this undesirable eiiect.

En accordance with the present invention, it has been discovered that if the end wall of the cathode ray tube is sumciently increased in thickness, the paths vof the' internally reiiected light rays will be so lengthened that they can be caused to fall outside the range of the screen. This increase in thickness may be producedby initially forming a tube or envelope dll with an extra thick end wall i2 as in the manner shown in Fig. 4. Alternatively, it may be produced as in the manner shown in Fig. l, by securing an additional transparent layer or block 20 in optical contact with wall l2 of a conventional cathode'ray tube i0, as by means of layer 22 of a suitable transparent adhesive..

parent plastic of suillcient hardness, such for example as methyl methacrylate. It may be formed of a. material having the same index of refraction as the glass forming wall I2, as in the embodiment shown in Fig. 1, but in the preferred embodiment of the invention, as will be pointed out below, it will comprise material having a lower index refraction than the glass used in conventional cathode' raytubes. Moreover, as is indicated by the dotted lines 25 in Fig. 1, layer 20 may if desired be so formed as to function as a lens to magnify the image on screen I4 or as part of a projection system therefor. v v

The adhesive used in layer 22 should secure ayer 2cm optical Contact with wan i2. If the index of refraction of layer 20 matches that of wall I2, then the index oi refraction of the adhesivein layer 22 should preferably match that of the' other layers. index of refraction of layer 20 does not match that of wall i2, then the index of refraction of the -adhesive may match that of either wall or layer 2t, and in any case it should not so dier from that of either of the other layers as to cause substantial reiiection.

It is believed that the operation of the embodi ment of the invention shown in Fig. 1 will be readily apparent from the ray diagrams therein. As in the case or the conventional cathode ray tube shown in Fig. 2, rays 2t, emanating from point i5 and incident upon the boundaries of wall i2 and layer 2@ at angles approaching the normal, are propagated without substantial re flection back towards the screen. Rays 2d correspond to rays id in Fig. 2, and represent the rays which in the conventional tube would suifer substantial internal reection at the outer surface of wall i2. However, if the indices of re fraction of wall i2 and layers 2li and 22 are sub= stantially matching, said rays are propagated beyond wall i2 without suffering substantial internal reflection until they reach the outer surface of layer 2d. At this point they will be ina ternally reiiected in the same manner as shown in Fig. 2, but owing to the fact that their paths have been lengthened by reason of their passage through layer 2G, after reiiection they will be directed on to the blackened sides of layer 2t and there absorbed without again striking screen it.

The construction shown in Fig. 1 is in general quite satisfactory for tubes of relatively small size. Furthermore, as has rbeen pointed out above, the same results may be obtained in the manner indicated in Fig. 4, by initially forming tube dll with an. extra 'thick end wall 42 on the inner side of which is fluorescent screen 44. It will be seen, however, that layer 20 will have to be increased in thickness if the size of the tube and screen is increased, in order to lengthen the paths of rays 28 suiiiciently so that when they are internally reflected they will not again reach screen M.

It should be understood that it is not essential to the operation of the invention that none of the reflected rays 28 reach the screen. yIf their paths are lengthened only to such an extent that when reected they reach the screen again at points relatively iar removed from that at which they originated, the result will still be a substantial improvement over the operation of the conventional device shown in Fig. 2, although for best results, the device of the invention should be so constructed that a minimum of reflected If, on the other` hand, thelight again reaches the screen. It may be stated u as a general rule, therefore, that the additional layer of material should be of such thickness and index of refraction that light emitted from the screen and incident on the outer'surface of the thickened wall at angles approaching the critical angle will be reflected in a direction which will intersect the surface defined by the screen at a point relatively remote from its point of origin thereon.

This general rule may also be stated mathematically. Referring to Fig. 1, for optimum results the minimum thickness of a layer 20 uniformly composed of the same material of index of refraction n may be expressed as follows:

It will be noted that in Fig. 1 angle i is assumed to be the critical angle for the material oi layer 2t, and that t is measured along a normal from the plane of the periphery of screen i6 to the point of incidence on the outer surface of layer 2li of a ray originating at the periphery of the screen and incident on the cuter surface of layer. 2@ at the critical angle. The distance r is sirn= ilarly measured on the plane oi the periphery of screen, it, but it will be understood that in the case oi tubes having a dat end wall, the screen and the plane of its periphery will coincide.

As a specific example of the above rule, if .it may beassumed that layer 2@ is composed or material" oi index of refraction substantially equal to ifi,k then the critical angle will be oi the order of 45, Accordingly, t should be not less than r and will preferably be somewhat greater, owing to the fact that there is substantial internal reiiection at angles approaching but less than the critical angle. Otherwise the purposes of the invention will be accomplished by changing the above inequalities to:

t=r cot i=r cot sin-ll/n r=t tan sin-ll/n It will be seen further that if layer 20 be composed of material of still lower index of refraction, the required thickness thereof will be reduced.

En the case especially of tubes having screens of relatively large size, preferred results will be obtained by `making the layer added to the tube of material of lower index of refraction than the material of the tube wall itself. An example of such construction is shown in Fig. 3, wherein element 30 represents the iront wall of a conventional cathode ray tube provided with. screen 32 on its inner surface, and havingsecured to its outer surface as by means of a suit able layer of adhesive 33 an additional layer 34 corresponding to layer 2li in Fig. l. Layer 34 is assumed to be of material of lower index of refraction than the glass in wall 30, and ex'- amples of suitable material for use therein iti-.- clude any of the relatively hard, transparent resins such as methyl methacrylate and similar materials. Adhesive layer 33 may have an index of refraction matching that of either wall 30 or layer 34, but preferably falling therebetween. It is, moreover, within the vscope of the invention to make layer 34 of material which will adhere to wall 30 without additional adhesive, or to use a relatively thick layer of adhesive. It may also be desirable to use an outer protective layer of glass or similar hard material, which need not be in optical contact with the remainder of the device.

The' advantage of the of construction upon the outer surface of layer 3 3 at angles approaching the critical angle traverse layer 34 more obliquely with respect to the optical axis of the system, and so for a given thickness of layer 34 are reflected back upon the screen at points farther removed from their point of origin than would be the case if the index of layer 34 were higher. This will be more readily apparent by reference to Fig. 3 wherein point 35 correspends to point I5 in Figs. 1 and 2, and rays 3G and 38 correspond to rays 28 and 28 respectively in Fig. 1. 'I'he course of rays 36 will be seen to correspond substantially to that of rays 23 in Fig.

1, with the exception of some slight degree of refraction takingplace as they pass from wall 30 and layer 33 into layer 34. Rays 33, on the other hand, are refracted to such an extent as they enter layer 34 that their lateral path within said layer is substantially lengthened, and thus when they are internally reiiected at its exterior surface they are directed towards points farther removed from that at which they originated than would be the c ase if layer 34 were of higher index than wall 30.

It will be seen from the above that by making layer 34 of material of lower index of refraction than wall 30, it is possible to produce satisfactory results with a layer 34 of less thickness than would be necessary if it were material of as high vindex of refraction as wall 30. At the same time,

low indices that such results are most easily ob-` tainable. An additional advantage derived from using material of low index in layer 34 lies in the fact that rays incident on its outer surface at angles approaching the normal will be transmitted with a minimum of reflection. Substantially the same advantages may'be'obtained in a construction such as that sh'own in Fig. 4 by making the entire tube, or at least the front wall 42, of material of relatively low index.

It should be: noted that in choosing material is that for optimum results, the minimum thick-r ness of each layer and of the entire structure canl be determined by the inequality r t1 tan sin-l llm+ta tansin-l 1lm+lstan sin-1 llm+ wherein t1, ta and t are the thicknesses respectively of layers 30, 33 and 34, and m, m and m are the respective indices of refraction of said layers. 'I'his relation may be expressed more generally for the case of a structure of g layers as follows:

2J, tan sin*x 1/`n, 1`

It should, noted that the various values for t in th'e above equations are measured along a normal drawn from the plane of the periphery of the screen to intersect at the outer surface angle. It should also be noted that it is assumed for use in layer 34 care should be takento avoid using a material having an index of refraction so 'widely different from that of wall 30'as to V cause internal reflection of rays 33 at'the inner surface'of layer 34. 'Ihe same factor should be considered in choosing .the proper adhesive for layer 33, and it will be seen that the'most satisfactory results will be obtained by using an adhesive of an index of refraction somewhere between that of wall330 and layer 34 in order to obtain` 'progressive refraction of rays 33. It

425 in Fig. l.

or 34 in th'e shape of a convex lens, satisfactory for the purpose of the above relations that a tangent to the point where said normal intersects with the outer surface of the outermost layer will be parallel to the plane of the periphery of the screen.

It should be understood that the invention is not to be construed as limited to any particular shape or size for layers 20 and 34 in wall 42. As has already been pointed out, for example, it is within the scope of the invention to make said elements in the shape of a lens, as indicated at Furthermore, by making layer 20 results may be obtained with `less material than is required if the outer surface of said layer is to be flat. Preferably sides 24 of layer 20 should be tapered as shown, and the degree oftaper is determined primarily'by'th'e width of the viewsides ofv layer 20 shouldbe considerably tapered, and the greater the extent to 'which sides 24 appreach' parallelism, the narrower Iwill' be the angle through which the 'entire surface of screenv y i4 will be visible. .y It should again be pointed tion is in no way to be considered aslimited to such embodiments thereof as prevent any light from being reflected back on to the screen whence it originates.A l In conventional cathode ray tubes it is to the llight 'rays *incident on4 the outer wall thereof? at *angles approaching vthe, critical angle which are-the chief cause of objectionable reflection.' If by following the teach`- ings of the invention'the construction of the tube is so modifled that said fraysarereilected onl to the screen at points relatively far removedfrom their `source randjfrorrlE the vpointsg'at 'which they would again 'reach'"th"screen in-"a conventional tube,"said` modification is' to' be consideredwfs fa111ng...withm lthe scape-'- 6r the; inventionieventhough itmayi-notfbe a'preferred embodiment" thereofu Many 'sucli "niodiilcatinswill doubt# less be apparent to :other-skilled' in' theV art',

are also to be considered as within the scope of thel invention.

tion than the layer adjacent thereto on theside nearer wall 33.

4' .livie general rule for constructions ot this type Since certain changes may be made in the above article and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter out that .the invencontained in the above. description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of" the invention which, as a matter of language, might be said to fall therebetween.

What is claimed is:

1. In a cathode ray tube having a wall thereof provided on its inner surface with a fluorescent screen, means for preventing undesirable light reflections, said means comprising a layer of transparent plastic material of an index of refraction less than that of the material of said wall, and a layer of adhesive bonding said plastic layer in optical contact with the outer surface of said wall, the index of refraction of said adhesive being less than that of the material of said wall and greater than that of said outer layer, the combined thickness of said wall and said layers being such that light emanating from a point on said screen and incident on the outer surface of said outer layer at angles approaching the critical angle thereof is reflected in directions which intercept the surface defined by said screen( at points relatively remote from the, point of origin thereon of said light.

2. In a cathode ray tube havinga wall thereof provided on its inner surface with a fluorescent screen, means for preventing undesirable light reflections, said means comprising a layer of transparent solid material in optical contact with the outer surface of said wall, said layer comprising a plurality of layers of transparent material, adjacent elements in said plurality being in optical contact with each other, adjacent layers having indices of refraction sufficiently close to prevent substantial internal reflection at the interfaces therebetween, said layers having progressively lower indices of refraction than that of the material of said wall, the combined thickness of said wall and said plurality of layers being such that light emanating from a point on said screen and incident on the outer surface of said outer layer at angles approaching the critical angle thereof is reflected in directions which intercept the surface defined by said screen at points relatively remote from the point of origin thereon of said light.

3. In a cathode ray tube having a wall thereof provided on its inner surface with a fluorescent screen, means for preventing undesirable light reflections, said means comprising a layer of transparent plastic material of an index of re`- fraction less than that of the material of said wall, and a layer of adhesive bonding said plastic layer` in optical contact with the outer surface of said wall, the index of refraction of said adhesive being less than that of the material of said wall and greater than that of said outer layer, the combined thickness of said wall and said layers being such that light emanating from a point on said screen and incident on the outer surface of said outer layer at angles approaching the critical 2,346,310 angle thereof is reflected against the sides of sai layer.

4. In a cathode ray tube having a wall thereof provided on its inner surface with a fluorescent screen, means for preventing undesirable light reflections, said means comprising a layer of transparent solid material in optical contact with the outer surface of said wall, said layer comprising a plurality of layers of transparent material, adjacent elements in said plurality being in optical contact with each other, adjacent layers having indices of refraction suillciently close to prevent substantial internal reflection at the interfaces therebetween, said layers having progressively lower indices of refraction than that of the material of said wall, the combined thickness of said wall and said plurality of layers being such that light emanating from a point on said screen and incident on the outer surface of said outer layer at angles approaching the critical angle thereof is reflected against the sides of said layer.

5. In a cathode ray tube having a wall thereof provided on its inner surface with a fluorescent screen, means for preventing undesirable light reflections, said means comprising a layer of transparent plastic material of an index ofrefraction less than that of the material of said wall, and a layer of adhesive bonding said plastic layer in optical contact with the outer surface of said wall, the index of refraction of said adhesive being less than that of the material of said wall and greater than that of said outer layer, the combined thickness of said wall and said layers being restricted by the relation r t| tan sin"l llm+h tan sin-l l/m+ta tan sin"l ilm+ wherein r is .the radius of the planeof the periphery of said screen, tr is the thickness of said tube wall, t2 is the thickness of said adhesive layer,

t; is the thickness of said plastic layer and m, n: and n3 are the respective indices of refraction of said layers. A

6.-In a cathode ray tube having a wall thereof provided on its inner surface with a fluorescent screen, means for preventing undesirable light reflections, said mean's comprising a layer of transparent solid material in optical contact with the outer surface of said wall, said layer comprising a plurality of layers of transparentv material, adjacent elements in said plurality being in optical contact with each other, adjacent layers having indices of refraction sulciently close to prevent substantial internal reflection at the interfaces therebetween, said layers having progressively lower indices of refraction than that of the material of said wall, the combined thickness of said wall and said plurality of layers being restricted by the relation fr Zvw 

